Hertz·sprung-Rus·sell diagram 
(hĕrts′sprŭng-rŭsəl)
(hĕrts′sprŭng-rŭsəl)n.
A graph of the absolute magnitudes or luminosities of stars plotted against their surface temperatures or colors, used to classify stars by their evolutionary stages.
[After Ejnar Hertzsprung (1873-1967), Danish astronomer, and Henry Norris Russell.]